Purpose

Have you ever needed to ignore a database column in ActiveRecord? For example, when you have a table with hundreds of millions of rows and you want to delete or rename a column with no down time. In order to pull that off you need to ignore the column on master, while you remove it on a slave. Once the column is deleted, you start replication to catch up to master before promoting the slave to master. Without a way to ignore a column on master you will get error when replicating.

Example

class Person < ActiveRecord::Base
   attr_ignore :last_name, :first_name
end